This chalk holder is meant for the Predator brand octagonal pool chalk. It is set up to be half the height of the chalk to make sure you can continue using it through more of the chalks life.

There is a recess in the bottom to hold a magnet (20mm diameter x 3mm thick neodymium N42 magnet). You will see one is glued in the bottom of the holder in the attached picture.

With the magnet (make sure you properly orient the N and S side appropriately) to work with the Kamui magnetic chalk holder belt clip.

I am working on a design that can be used instead of the Kamui magnetic belt clip. Will post that when it is ready.

Note that predator chalk doesn't seem to be uniform, so you may need to rotate it to get it to fit properly.